Electric motor excited by permanent magnets

ABSTRACT

The rotor ( 7 ) of an electric motor is provided, on its surface and/or in its interior, with recesses ( 12, 13, 14, 15, 16, 17, 18, 19 ), which are placed and dimensioned such that they do not interrupt lines of the magnetic flux that are oriented radially to the surface of the rotor ( 7 ) and form a short circuit via the rotor ( 7 ). These recesses reduce the thermal capacity of the rotor ( 7 ) to such an extent that a layer of a magnetic material that forms a plurality of permanent magnets ( 8, 9, 10, 11 ) can be sprayed onto the surface of the rotor ( 7 ).

PRIOR ART

[0001] The present invention relates to an electric motor with permanent-magnet excitation, which has a rotor on whose surface, oriented toward the inside of a stator, a plurality of permanent magnets are disposed.

[0002] A permanent-magnet-excited electric motor of this kind is known for instance from U.S. Pat. No. 6,204,587 or German Patent DE 3 844 074 C2. The rotors of the electric motors described in these publications have permanent magnets on their surface, and adjacent permanent magnets have opposite polarity. As taught in particular by DE 3 844 074 C2, especially in high-speed motors, provisions must be made to absorb the centrifugal forces acting on the permanent magnets. To that end, a massive hollow cylinder is slipped onto the rotor, in such a way that it rests with its inner wall directly on the permanent magnets. This fixes the permanent magnets in their position on the rotor, and thus a fastening of the permanent magnets that is reliable even at very high rpm is achieved. Such relatively complicated provisions for fixation of the permanent magnets are required quite particularly whenever the permanent magnets have a great wall thickness, so that they can be magnetized not radially but laterally, so that a magnetic short circuit through the rotor can be dispensed with. Permanent magnets with a great wall thickness are typically made by pressing from a magnetic material. Pressed magnetic material has only slight mechanical tensile strength, however, so that bandaging of the permanent magnets is required as disclosed for instance in DE 3 844 074 C2.

[0003] The object of the invention is to disclose an elm of the type defined at the outset in which the magnetic material forming the permanent magnets can be applied in a relatively thin layer directly onto the surface of the rotor.

ADVANTAGE OF THE INVENTION

[0004] This object is attained with the characteristics of claim 1, in that the rotor is provided, on its surface and/or in its interior, with recesses, which are placed and dimensioned such that they do not interrupt lines of the magnetic flux that are oriented radially to the surface of the rotor and form a short circuit via the rotor.

[0005] The coefficients of thermal expansion of the iron of the rotor and of the magnetic material in fact differ quite sharply, and the high thermal capacity of the rotor leads to rapid cooling of the magnetic material applied to the rotor, for instance by spraying. As a result, there is the danger that the applied magnetic material will tear. This unwanted effect can be avoided by the provision according to the invention for reducing the thermal capacity of the rotor, by making recesses in the rotor. Since precisely the exciter flux, which extends radially outward from the permanent magnets of the rotor to the air gap between the rotor and the stator, increases the available torque of the motor, the recesses are placed such that they precisely do not interrupt the lines of the magnetic flux in the radial direction to the surface of the rotor.

[0006] Advantageous refinements of the invention are disclosed in the dependent claims.

[0007] Since the lines of the magnetic flux that are oriented tangentially or virtually tangentially to the surface of the rotor and that form a short circuit via the rotor reduce the available torque of the rotor, the recesses are placed and dimensioned such that they interrupt lines of the magnetic flux oriented tangentially or nearly tangentially to the surface of the rotor.

[0008] The permanent magnets advantageously comprise a layer of a plastic-bonded magnetic material sprayed onto the surface of the rotor.

[0009] It is expedient that for recesses that extend in the direction of the longitudinal axis of the rotor to be let into the surface of the rotor, in the region between each two permanent magnets of opposite polarity. The centrifugal strength of the rotor can be increased by providing that the recesses have a dovetail cross section. This is because the dovetail shape of the recesses allows wedging of the permanent magnets, so that they are secured not only against a rotary motion but also against a radial motion.

[0010] In addition to the recesses let into the surface of the rotor, or solely in the interior of the rotor, at least one recess can be provided below each transition between two adjacent opposite-polarity permanent magnets, which recess has the form of a slot, which extends in the direction of the longitudinal axis of the rotor and expands transversely to the longitudinal axis of the rotor. Recesses in the interior of the rotor lead on the one hand to a very sharp reduction in the thermal capacity of the rotor, and form ideal barriers to components of the magnetic flux that are oriented tangentially to the surface of the rotor.

[0011] The recesses located in the interior of the rotor can be utilized for placing a material in them that compensates for an imbalance of the rotor.

[0012] The rotor advantageously comprises layered iron sheets.

DESCRIPTION OF THE EXEMPLARY EMBODIMENT

[0013] The sole FIGURE of the drawing shows a cross section through an electric motor, whose stator 1 is shown only in part, with two poles 2, 3 and the associated windings 4, 5. A rotor 7 disposed on a motor shaft 6 is supported rotatably inside the stator 1. The rotor 7 preferably comprises a stack of iron sheets layered on one another. The stator 1 can also comprise iron sheets layered on one another. The rotor 7 and the stator 1 are preferably stamped out of a stack of metal sheets in a single operation.

[0014] The electric motor shown only in part is a permanent-magnet-excited three-phase synchronous motor or a brushless direct-current motor. The rotor 7 is therefore coated on its surface with permanent magnets 8, 9, 10, and 11. In the exemplary embodiment shown, in order to make four magnet poles, four permanent magnets 8, 9, 10, and 11 are applied to the surface of the rotor 7; permanent magnets adjacent to one another are magnetized, as indicated by arrows, in opposite directions radially to the surface of the rotor 7. The four permanent magnets 8, 9, 10, 11 are formed by a layer of a preferably plastic-bonded magnetic material (such as NdFeB) sprayed directly onto the surface of the rotor 7, which surface together with the inside of the stator 1 forms an air gap. The iron rotor 7 forms the short-circuit yoke for the magnetic flux of the permanent magnets 8, 9, 10, 11.

[0015] The coefficients of thermal expansion of the magnetic material and of the iron material of the rotor 7 are quite different from one another. Moreover, the thermal capacity of the rotor 7, if the rotor comprises a solid material, is quite high. For the magnetic material sprayed onto the rotor 7, there is therefore the risk, from overly rapid cooling because of the high thermal capacity of the rotor 7 located under it, that the sprayed-on magnetic layer will tear. To prevent this, the thermal capacity of the rotor yoke is reduced very sharply, by providing that recesses 12, 13, 14, 15 are let into the surface of the rotor 7 and/or recesses 16, 17, 18, 19 are let into the interior of the rotor 7. These recesses 12-19 bring about a sharp reduction in the thermal capacity of the rotor material, thereby preventing the magnetic material sprayed onto the rotor 7 from cooling overly rapidly and tearing as a result.

[0016] The recesses 12, 13, 14, and 15 let into the surface of the rotor 7, which extend in the direction of the longitudinal axis of the rotor 7, perpendicular to the plane of the drawing, are placed below the transitions between adjacent, opposite-polarity permanent magnets 8, 9, 10, 11. As disclosed in “Berichte aus dem Institut für elektrische Maschinen und Antriebe” [Reports from the Institute for Electrical Machines and Drive Mechanisms], Vol. 7, Volker Bosch: Elektronisch kommutiertes Einzelspindelantriebssystem [Electronically Commutated Single-Spindle Driver System], Shaker Verlag, Aachen 2001, this placing of the recesses is chosen so that lines of the magnetic flux oriented tangentially or nearly tangentially to the surface of the rotor and forming a short circuit via the rotor are interrupted by these recesses. As a result, it is attained that lines of flux oriented tangentially in the air gap between the rotor 7 and the stator 1, which weaken the torque of the motor, are blocked. Conversely, the magnetic flux lines oriented radially to the surface of the rotor 7 are not blocked, because they increase the available torque of the motor. In accordance with these aspects, the recesses 16, 17, 18, 19 let into the interior of the rotor 7 and also extending in the direction of the longitudinal axis of the rotor 7 are placed and dimensioned below the transitions between adjacent, opposite-polarity permanent magnets 8, 9, 10, 11.

[0017] An optimal barrier for magnetic flux lines extending tangentially or nearly tangentially to the rotor surface is formed by recesses 16, 17, 18, 19, which have a slotlike cross section extending transversely to the rotor longitudinal axis. Instead of providing only one recess 16, 17, 18, 19 below each transition between two adjacent permanent magnets, as shown here in the drawing, a plurality of slotlike recesses, staggered radially, for instance, can also be let into the rotor 7.

[0018] The centrifugal resistance of the rotor can be increased by providing that the recesses 12, 13, 14, and 15 let into the surface of the rotor 7 have a dovetail cross section. This is because the dovetail shape of the recesses 12, 13, 14, and 15 enables wedging of the permanent magnets 8, 9, 10, and 11, so that they are secured not only against a rotary motion but also against a radial motion.

[0019] Nonmagnetic material (“balancing cement”) can be placed in the recesses 16, 17, 18, 19 that are let into the interior of the rotor 7 in particular, in order to compensate for an imbalance of the rotor 7. 

1. An electric motor with permanent-magnet excitation, which has a rotor (7) on whose surface oriented toward the inside of a stator (1) a plurality of permanent magnets (8, 9, 10, 11) are disposed, characterized in that the rotor (7) is provided, on its surface and/or in its interior, with recesses (12, 13, 14, 15, 16, 17, 18, 19), which are placed and dimensioned such that they do not interrupt line of the magnetic flux oriented radially to the surface of the rotor (7) and forming a short circuit via the rotor (7).
 2. The electric motor of claim 1, characterized in that the recesses (12, 13, 14, 15, 16, 17, 18, 19) are placed and dimensioned such that they interrupt line of the magnetic flux oriented tangentially or nearly tangentially to the surface of the rotor (7) and forming a short circuit via the rotor (7).
 3. The electric motor of claim 1, characterized in that the permanent magnets (8, 9, 10, 11) comprise a layer of a plastic-bonded magnetic material sprayed onto the surface of the rotor (7).
 4. The electric motor of claim 1 or 2, characterized in that recesses (12, 13, 14, 15) that extend in the direction of the longitudinal axis of the rotor (7) are let into the surface of the rotor (7), in the region between each two permanent magnets (8, 9, 10, 11) of opposite polarity.
 5. The electric motor of claim 4, characterized in that the recesses (12, 13, 14, 15) have a dovetail cross section.
 6. The electric motor of one of claims 1 or 2, characterized in that in the interior of the rotor (7), there is at least one recess (16, 17, 18, 19) below each transition between two adjacent opposite-polarity permanent magnets (8, 9, 10, 11), which recess has the form of a slot, which extends in the direction of the longitudinal axis of the rotor (7) and expands transversely to the longitudinal axis of the rotor (7).
 7. The electric motor of one of claims 1 or 6, characterized in that a material that compensates for an imbalance of the rotor (7) is placed in one or more of the recesses (16, 17, 18, 19) located in the interior of the rotor (7).
 8. The electric motor of one of the foregoing claims, characterized in that the rotor (7) comprises layered iron sheets. 